Optical in situ calibration of Sb to grow disordered GaInP by MOVPE

Reflectance anisotropy spectroscopy (RAS) was employed to determine the optimal specific molar flow of Sb needed to grow GaInP with a given order parameter by MOVPE. The RAS signature of GaInP surfaces exposed to different Sb/P molar flow ratios were recorded, and the RAS peak at 3.02 eV provided a feature that was sensitive to the amount of Sb on the surface. The range of Sb/P ratios over which Sb acts as a surfactant was determined using the RA intensity of this peak, and different GaInP layers were grown using different Sb/P ratios. The order parameter of the resulting layers was measured by PL at 20 K. This procedure may be extensible to the calibration of surfactant-mediated growth of other materials exhibiting characteristic RAS signatures

Highly conductive p++-AlGaAs/n++-GaInP tunnel junctions for operation up to 15,000 suns in concentrator solar cells

In the last few decades there has been great interest in III-V multijunction solar cells (MJSC) for concentrator applications due to their promise to significantly reduce the cost of electricity. Being formed by series connection of several solar cells with different bandgaps, a key role in a MJSC structure is played by the tunnel junctions (TJ) aimed to implement such series connection. Essentially, tunnel junctions (tunnel diodes or Esaki diodes) are thin, heavily doped p-n junctions where quantum tunneling plays a key role as a conduction mechanism. Such devices were discovered by Nobel laureate Leo Esaki at the end of 1950. The key feature of tunnel junctions for their application in MJSC is that, as long as quantum tunneling is the dominant conduction mechanism, they exhibit a linear I-V dependence until the peak tunneling current (Jp) is reached. This initial ohmic region in the I-V curve is ideal for implementing low-loss interconnections between the subcells with different energy bandgaps that constitute a MJSC

Highly conductive p++-AlGaAs/n ++-GaInP tunnel junctions for ultra-high concentrator solar cells

Tunnel junctions are key for developing multijunction solar cells (MJSC) for ultra-high concentration applications. We have developed a highly conductive, high bandgap p  + + -AlGaAs/n  + + -GaInP tunnel junction with a peak tunneling current density for as-grown and thermal annealed devices of 996 A/cm 2 and 235 A/cm 2, respectively. The J–V characteristics of the tunnel junction after thermal annealing, together with its behavior at MJSCs typical operation temperatures, indicate that this tunnel junction is a suitable candidate for ultra-high concentrator MJSC designs. The benefits of the optical transparency are also assessed for a lattice-matched GaInP/GaInAs/Ge triple junction solar cell, yielding a current density increase in the middle cell of 0.506 mA/cm 2 with respect to previous designs

Effect of Sb on the quantum efficiency of GaInP solar cells

The energy bandgap of GaInP solar cells can be tuned by modifying the degree of order of the alloy. In this study, we employed Sb to increase the energy bandgap of the GaInP and analyzed its impact on the performance of GaInP solar cells. An effective change in the cutoff wavelength of the external quantum efficiency of GaInP solar cells and an effective increase of 50 mV in the open-circuit voltage of GaInP/Ga(In)As/Ge triple junction solar cells were obtained with the use of Sb. Copyright © 2016 John Wiley & Sons, Ltd

On the use ofSb to improve the performance of GaInP subcells of multijunction solar cells

GaInP is a material commonly employed for the top subcells of different multijunction solar cells architectures. In this study, the performance of GaInP top cells has been improved by increasing the energy band gap with the use of Sb as a surfactant during the MOVPE growth of the structures. The optimization of the appropriate Sb molar flow was done by Reflectance Anisotropy Spectroscopy. Different characterization techniques have been employed to assess the effect of Sb on the morphology, microstructure and optoelectronic properties of the resulting GaInP grown with different Sb/P ratios. Finally, the performance of several GaInP subcells with different order parameters has been assessed

Structural challenges in multijunction solar cells for ultra-high CPV

The paths towards high efficiency multijunction solar cells operating inside real concentrators at ultra high concentration (>1000 suns) are described. The key addressed factors comprehend: 1) the development of an optimized tunnel junction with a high peak current density (240 A/cm2) to mitigate the non-uniform light profiles created by concentrators, 2) the inclusion of highly conductive semiconductor lateral layers to minimize the effects of the non-uniform light profiles in general, and the chromatic aberration in particular; and 3) an adequate design of reliability studies to test multijunction solar cells for real operation conditions in order to determine the fragile parts in the device and improve them. These challenges are faced by means of experimental and theoretical investigation using a quasi-3D distributed circuital model

Characterization of doped GaInP nanowires for photovoltaics (Caracterización de nanohilos the GaInP con distintos dopados para aplicaciones en energía fotovoltaica)

The interest in renewable energy sources has increased recently, which has resulted in increasing research in solar energy as an environmentally friendly way to obtain electricity. Direct harvesting of solar energy to electricity is called photovoltaics, where III-V semiconductors nanowires can be used to fabricate multijunction solar cells with promise to deliver high efficiency at low cost. The division of Solid State Physics at Lund University has been working during the last few years on a project for the fabrication of a tandem p-n junction solar cell. In this thesis project, doped nanowires of GaInP have been investigated and evaluated with optical and electrical measurements, with the aim to create high-bandgap p-n junctions. Electrical measurements show that both p- and n-doping can be achieved. Nanowires with p-i-n doping show excellent rectification with reasonably low ideality factors, and generate a clear photocurrent under illumination. Under forward bias, the p-i-n devices give yellow electroluminescence in agreement with photoluminescence experiments.[RESUMEN] El interés en las energias renovables ha crecido recientemente resultando en una gran motivación por la energía solar, la cual no causa gran impacto medioambiental a la hora de obtener electricidad. En relación con la energía fotovoltaica, nanohilos de materiales semiconductores del grupo III-V pueden ser usados para fabricar multiuniones de células solares prometiendo una mayor eficiencia y un bajo coste. El departamento de Física del Estado Sólido de la Universidad de Lund ha estado trabajando durante los últimos años en el projecto AMON-RA con el fin de obtener una nueva tecnología basada en la obtención de una célula solar tipo tándem con el uso de nanohilos semiconductores del grupo III-V. En este projecto de Máster, uniones p-n de nanohilos de GaInP dopados han sido investigados y caracterizados con medidas ópticas y eléctricas con el fin de obtener un alto gap de energía prohibida necesario en este projecto AMON-RA mencionado anteriormente.Contactos p y n han sido capaz de ser fabricados y estudiados. Uniones p-i-n han sido caracterizadas obteniendo una excelente curva de rectificación y fotocorriente al iluminar el dispositivo, con un razonable factor de idealidad. Por otro lado, la aplicación de voltaje en el dispositivo ha dado lugar a la emisión de luz en el rango amarillo, resultado que concuerda con las medidas de fotoluminiscencia obtenidas

Optimization pathways to improve GaInP/GaInAs/Ge triple junction solar cells for CPV applications

La tecnología de concentración fotovoltaica (en inglés, Concentration Photovoltaics, CPV) ha experimentado un intenso desarrollo desde principios de los años 2000. En particular, las células solares de triple unión (GaInP/GaInAs/Ge) ajustadas en red siguen dominando el mercado CPV. Esta tesis pretende contribuir en la investigación de este tipo de célula multiunión desarrollada previamente en el Grupo de Semiconductores III-V del Instituto de Energía Solar de la Universidad Politécnica de Madrid (IES-UPM). Los distintos aspectos abordados para la mejora de la eficiencia de esta estructura comienzan desde el substrato de Ge, que sustenta el resto de la estructura semiconductora, pasando por distintos aspectos de optimización de la estructura semiconductora, hasta la incorporación de grafeno como electrodo transparente en la parte superior del dispositivo. Como primer punto, se aborda el crecimiento epitaxial de semiconductores III-V sobre substratos de Ge. El estudio de la contaminación por Ge en las distintas capas semiconductoras es de especial interés, ya que una elevada concentración no intencionada de este material en las capas activas afecta negativamente a sus propiedades. En este estudio se presta especial atención tanto a la incorporación de Ge por difusión en fase sólida en las primeras capas semiconductoras crecidas, como a su incorporación desde la fase gaseosa durante el crecimiento mediante la epitaxia en fase vapor a partir de precursores metalorgánicos (en inglés, Metalorganic Vapour Phase Epitaxy, MOVPE) de las distintas capas. Para ello, se analiza la influencia de distintas condiciones y parámetros de crecimiento para encontrar el mejor proceso epitaxial para mitigar este efecto. Continuando con el análisis de mejora del dispositivo, hemos abordado los tres aspectos más destacados que hemos detectado para la mejora de la célula solar de triple unión. En primer lugar, con motivo de la carga térmica acumulada y soportada por la subcélula de germanio durante el crecimiento del resto de las estructura, la VOC resultante de ésta es menor de lo esperado. Con la intención de disminuir dicha carga térmica y mejorar la VOC de la subcélula de germanio, hemos optimizado el proceso de nucleación. En particular, en esta tesis se presenta una nueva rutina de nucleación de la capa de nucleación de GaInP y de la capa buffer de GaInAs poniendo especial interés en la reducción de la carga térmica como consecuencia de disminuir el espesor de estas capas, además de la bajada en temperatura a la que se crece la buffer de GaInAs. El siguiente aspecto a analizar es la mejora de la subcélula de GaInP. Continuando con el uso de Sb como elemento surfactante durante el crecimiento de la base del GaInP, en esta tesis hemos desordenado aún más el GaInP con el uso de dicho surfactante tanto en el crecimiento epitaxial de la base como del emisor, con el objetivo de aumentar la banda de energía prohibida (band gap) de esta subcélula y por tanto su VOC. En paralelo, hemos realizado la optimización la capa ventana de AlInP modificando ligeramente su contenido en aluminio y aumentando su dopado con el objetivo de minimizar la absorción de luz en dicha capa. Por otro lado, con motivo de la integración de una nueva unión túnel entre la subcélula de GaInP y la subcélula de GaInAs, hemos detectado una resistencia serie elevada a altas concentraciones luminosas. Ya que esta unión túnel presentaba unas excelentes propiedades optoelectrónicas antes de integrarse en la célula solar, la resistencia serie elevada sugiere una mala hetereounión entre las capas vecinas (especialmente de la capa ventana de la subcélula de GaInAs) y esta nueva unión túnel. Con el objetivo de reducir dicha resistencia hemos analizado distintos niveles de dopaje e incluso el uso de otros materiales en la capa ventana. El desarrollo de estas mejoras se completa con su integración en célula completa de triple unión con el objetivo de estudiar su impacto final. La mejor célula de triple unión desarrollada previamente en el grupo se usa como referencia epitaxial (capas, dopados, etc). A esta estructura se van incorporando gradualmente las diversas mejoras produciendo diversas generaciones de dispositivos que son analizados y comparados mediante diversas técnicas de caracterización (curvas I-V, eficiencia cuántica, etc) con la célula de referencia. Como resultado de esta integración se han logrado eficiencias superiores al 40%. No obstante, se podrían haber logrado mayores eficiencias si todas las mejoras parciales hubieran sumado todo su potencial teórico. Se presenta un análisis de pérdidas para apuntar las causas de esta integración no óptima. Como último aspecto desarrollado a lo largo de esta tesis doctoral, hemos abordado el estudio del potencial del grafeno para su integración en una estructura fotovoltaica multiunión de semiconductores III-V. El principal objetivo de la integración de este novedoso material es la mejora de extracción de corriente del dispositivo trabajando a altas concentraciones (disminución de la resistencia serie y, por tanto, mejora del factor de forma). Primeramente, hemos caracterizado sus propiedades tanto ópticas como eléctricas que nos permitan una implementación adecuada en una célula multiunión. Posteriormente, hemos realizado la transferencia e integración de grafeno en una célula de triple unión así como su análisis y comparación respecto al mismo tipo de célula sin grafeno. Uno de los resultados de este trabajo ha sido una solicitud de patente sobre células solares multiunión de semiconductores III-V que incorporen grafeno. ----------ABSTRACT---------- Concentration Photovoltaic (CPV) technology has been growing up intensively since the early 2000s. In particular, GaInP/GaInAs/Ge lattice matched triple junction solar cells are still dominating the CPV market. This thesis has focused on the investigation of this kind of multijunction solar cells aiming at further developing the basic technology previously developed in the III-V Semiconductor Group at the Solar Energy Institute of the Technical University of Madrid (IES-UPM). The different aspects that have been tackled in order to enhance the efficiency of this structure begin with the Ge substrate, which supports the rest of the structure, up to the incorporation of graphene on top of the device to serve as a transparent electrode. Firstly, the epitaxial growth of III-V materials on Ge substrates is considered. Ge contamination along the semiconductor layers is of special interest, since the non-intentional incorporation of Ge in the active layers will have a detrimental effect on their properties. In this study, attention has been paid not only to the Ge solid-state diffusion that takes place at the first epitaxial layers next to the III-V/Ge heterointerface, but also to the incorporation of Ge from the gas phase during the epitaxial growth by Metalorganic Vapor Phase Epitaxy (MOVPE). Thus, the influence of different conditions and growth parameters are analyzed with the purpose of finding the best combination of process parameters to mitigate this effect. Following with the device optimization, three main aspects detected as possible improvement mechanisms had been addressed. On the one hand, the VOC of the Ge bottom cell is lower than expected, due to the accumulated thermal load of the growth of the rest of the MJSC structure. With the aim of mitigating such thermal load and improving this VOC, several III-V on Ge nucleation strategies are studied. In particular, this Thesis presents a new nucleation routine of the GaInP nucleation layer and the GaInAs buffer layer where special attention has been paid to minimize their contribution to the thermal load in terms of thickness (i.e. growth time) and temperature. The following step is the optimization of the GaInP subcell. Ensuing with the use of Sb as surfactant during the growth of the GaInP base, in this Thesis the GaInP subcell is further disordered (i.e. its energy band gap is increased) with the use of Sb in both the base and emitter. The goal of this step is to increase the energy band gap of the GaInP subcell and thus its VOC. Additionally, the AlInP window is optimized by modifying its Al composition as well as increasing its doping level with the aim of minimizing the light absorption in this layer. In past works, we developed highly transparent high bandgap GaInP/AlGaAs tunnel junctions (TJ) with excellent performance as isolated devices. However, when such tunnel junctions were integrated between the GaInP and GaInAs subcells in complete triple-junction solar cells, we detected the presence of a non-negligible series resistance. In this thesis, we have analyzed this integration problem and found that it might be related to the heterejunction between the new TJ and the adjacent layers (specially, the window layer of the GaInAs subcell).With the aim of decreasing such series resistance, different doping levels as well as the use of other materials at this window layer were studied and the integration problem has been sorted out. The implementation on a complete TJSC of these partial developments are consequently undertaken in order to evaluate their joint final impact. The best TJSC structure previously developed in our group is used as a reference and several new designs gradually introducing the partial improvements studied are analyzed and compared by different characterization techniques (I-V curves, external quantum efficiency…). The outcome of this integration has been the achievement of solar cell efficiencies in excess of 40%. However, higher efficiencies would have been reached if all the individual improvements had added their full potential. A loss analysis is presented to figure out the cause behind this non-optimum integration. As the last part of this Thesis, the potential of graphene for its integration in a multijunction solar cell is considered. The main objective behind the use of this new material is to enhance the photocurrent extraction of the device when working at very high concentration levels (by reducing the lateral series resistance and consequently improving the fill factor). Several analyses on its optical and electrical properties are presented in order to allow for an adequate implementation in a multijunction solar cell. Finally, graphene layers are transferred and integrated in a multijunction solar cell and the performance of the resulting devices is compared with similar devices without graphene. As a result of this work, a patent on III-V semiconductor multijunction solar cells incorporating graphene has been filed

Assessment of the energy yield gain in high CPV systems using graphene-enhanced III-V multijunction solar cells

It was recently shown that the use of graphene as a transparent electrode deposited between the antireflection coating and the front grid of a solar cell can help mitigate resistive losses. Graphene-enhanced solar cells can be manufactured by retrofitting existing off-the-shelf multijunction solar cell designs, making its incorporation into HCPV systems transparent for the module manufacturer. This paper presents a theoretical evaluation of yearly energy yield gains in HCPV systems equipped with graphene-enhanced solar cells as compared with the same system using conventional triple-junction solar cells. These calculations show that the energy production gain when using graphene-enhanced modules may exceed 6% in reasonable locations for CPV (Fes, Madrid, Nicosia) and clearly exceed 7% in a good location such as Phoenix, whereas graphene integration cost would increase the €/Wp merit figure in 2-3%